Rolling lines for strip are known which, in order to produce more than 800,000/1,000,000 tons/per year, start from the continuous casting of slabs and using continuous finishing trains with several rolling stands.
If thick slabs are cast, from 130 mm or more in thickness, the continuous finishing train is preceded by a reversing roughing train, whereas if the starting slab is a thin slab, with a thickness of less than 130 mm, for direct rolling, the train is formed simply by 5/9 continuous stands without a roughing train. For productions of less than 800,000/1,000,000 tons/per year a Steckel rolling mill with one or more reversing stands is commonly used, normally fed with slabs having a thickness from 150 to 250 mm.
A rolling line starting from thick slabs normally provides step-wise heating furnaces, a high pressure water de-scaler, a cropping shear, a Steckel reversing rolling train with one or two stands, a laminar cooling system and a winding unit.
Instead, a rolling line starting from thin slabs typically provides a casting machine of thin slabs, a system for the restoration, maintenance or homogenization of the temperature of the cast material, for example a tunnel furnace, a high pressure water de-scaler, a Steckel reversing rolling train with one or two stands, a laminar cooling system and a winding unit.
The rolling plant which starts from thin slabs, compared to that which starts from thick slabs, normally allows a saving, due to the fact that the cropping shear is not required, that the Steckel rolling stand or stands can have smaller diameters of the work rolls, about 740 mm instead of 810 mm: given the same compression, this allows to use rolling forces lower by 20-30%, with subsequent reductions in the sizes of the machine. Moreover, lower rolling forces also produce reduced rolling torque, and the size of the main motors will consequently have a smaller torque value, even less than 15-20%.
It is also known that rolling plants with reversing rolling trains of the Steckel type with one or more stands which use a slab with a thickness from 150 to 250 mm or more have limitations in productivity, in minimum thickness obtainable and in dimensional and surface quality of the final strip; the productivity is limited, given the great thickness of the starting slab, by the high number of rolling passes through the stand or stands and consequently by the long inversion down-times, with consequently long overall times from the beginning to the end of rolling; this also determines a lack of homogeneity of temperature along the strip, a high temperature loss and the formation of scale which negatively affect the final quality of the strip produced.
Moreover, the high temperature loss makes it impossible to roll thin slabs of finished product, for example from 1.8 to 1.2 mm or less.
Finally, the surface quality of the finished product is also affected by the use of the work rolls for the numerous passes of the cold head and tail ends and the consequent rapid deterioration of the surface of the rolls themselves. In order to reduce this disadvantage it is necessary to change the work rolls frequently, with consequent stoppages, compromising the factor of use and productivity of the plant.
A rolling line is known from document EP-A-0.625-383, consisting of a casting machine able to cast a slab of about 50 mm in thickness, a shearing unit, an inductor furnace, a tunnel furnace, a de-scaler, a two-stand rolling unit of the reversing type, or a continuous type with five stands in line, a cooling unit and a winding unit. The two-stand reversing rolling unit determines a reduction in thickness of the slab to a desired final value of about 1.5-2 mm by means of three double rolling passes. In this known solution, the thickness of the slab entering the reversing rolling unit is the same constant thickness of the slab which is cast. In this way, the known line is not adaptable according to the final thickness and width of the strip and of the type of steel, in order to obtain the final product with a minimum number of passes, because the thickness of the slab entering the reversing rolling unit cannot always be the ideal one; it is thus necessary to modify the thickness of the cast slab, which negatively influences the stability of the casting process. Moreover, in order to minimize the number of rolling passes, the known line has to have a high casting speed and therefore much more stressed working conditions.
Other casting lines and methods are disclosed in EP-A1-937.512, U.S. Pat. No. 4,675,974 and U.S. Pat. No. 6,182,490.
None of these documents, like EP'383, disclose the provision of a forming or roughing stand positioned immediately downstream the casting machine. The only forming or roughing stand provided upstream the Steckel rolling mill is disclosed in EP'512, but in this case the forming stand is located downstream the furnace, therefore not immediately downstream the casting machine. Moreover, the reduction provided in the roughing stand of EP'512 is designed to be up to 50%. None of these documents, therefore, allow to maintain low the number of sequential passes in the Steckel rolling mill for all the range of thicknesses that can be produced by the rolling line.
One purpose of the present invention is to achieve a rolling line with a Steckel rolling train with two reversing stands, and to perfect a relative method, which allows to reduce to a minimum the number of rolling and inversion passes and therefore reduce the total rolling time, with consequent increase in the productivity of the rolling mill, for the whole range of thicknesses that can be produced by the rolling line.
Another purpose is to reduce to a minimum the number of rolling passes, without imposing very stressed working conditions on the line, in particular with regard to the casting speed.
Another purpose of the present invention is to obtain a greater uniformity/homogeneity of the temperature along the strip being rolled and a lower overall temperature loss.
Another purpose is to increase the factor of use of the plant, increasing the working life of the work rolls.
Furthermore, another purpose of the present invention is to exploit to the utmost the great plasticity of the steel at the high temperatures which it has just after it has solidified, to carry out the roughing rolling of the product emerging from the continuous casting machine, so that it is thus possible to use smaller stands and hence with less power installed and with a considerable energy saving. The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.